The present invention relates to the field of data transmission and reception and, in particular, to the interception of spread-spectrum data transmissions.
Spectrum spreading is a technique used to make it difficult to intercept a transmitted signal from a radar transmitter or the like at a distance The spread-spectrum technique involves coding the transmitted signal so the bandwidth of the signal is many times greater than the bandwidth of the information transmitted. Thus, the spectral density of the signal energy is much less than it was before coding and an unauthorized receiver at a distance will be presented with a signal immersed in background noise. An authorized receiver, on the other hand, applies a decoding algorithm to the received noisy signal and compresses the transmitted information back into its original bandwidth, where it will stand out from the noise.
One standard method of coding a signal consisting of a series of pulses is the method of frequency hopping. This method involves changing a center frequency of each pulse in a prearranged pattern, so that the total bandwidth occupied by the pulse train is very large (or, conversely, so that a particular portion of the band is occupied by a pulse very infrequently). Frequency hopping is relatively simple to implement, and yields results generally considered to be approximately equivalent to more complex methods of spectrum spreading. In a frequency-hopped signal, the frequency spectrum of a single pulse "fills" a portion of the frequency domain with a (sin x)/x distribution. A series of these distributions side-by-side provides a spectrum that is more-or-less uniform.
The detection of a signal by a receiver depends on the relative energy of the received signal and the competing noise. The signal energy depends on the strength of the transmitted signal, the distance R between the transmitter and the receiver, and the gains of the transmitting and receiving antennas. The noise energy depends on the internal noise of the receiver, natural noise sources, e.g., galactic noise, and man-made noise sources, e.g., jammers.
The ideal detector is called a "matched filter." A matched filter concentrates the energy of the received signal (the group of hopped pulses) into a single pulse while passing through as little noise as possible. The output of the detector is a pulse embedded in a stream of noise. The strength of the pulse relative to the noise is characterized by the ratio: EQU S=E/N,
where S is a constant value for each predetermined distance of the receiver from the transmitter, E is the energy of the set of hopped pulses expressed in joules, and N is the "noise power density" or power per unit bandwidth of the noise expressed in watts per cycle per second (joules).
A conventional wideband receiver searching for set of hopped pulses does not act as a matched filter. If the transmitted pulses are hopped over a bandwidth that is x times the bandwidth of a single pulse, x times as much noise is introduced. Thus the receiver must approach quite close to the transmitter before a point is reached where an individual pulse is strong enough that it will stand out from the noise, i.e., S=E/N will be large enough to provide an adequate probability of signal detection with an acceptably small false alarm rate.
Thus, because frequency hopped signals appear to have the same spectral characteristics as general spread-spectrum signals to sufficiently distant transmitters, they are difficult to intercept and decode without knowledge of the frequency hopping scheme.
It would be advantageous to be able to intercept such frequency hopping signals by a receiver that does not possess the frequency hopping scheme at distances where the hopped signal is at least partially immersed in noise.